Role of Naturally Occurring Gas Hydrates in Sediment Transport

Abstract

Naturally occurring gas hydrates have the potential to store enormous volumes of both gas and water in semi-solid form in ocean-bottom sediments and then to release that gas and water when the hydrate's equilibrium conditions are disturbed. Therefore, hydrates provide a potential mechanism for transporting large volumes of sediments. Under the combined low bottom-water temperatures and moderate hydrostatic pressures that exist over most of the continental slopes and all of the continental rises and abyssal plains, hydrocarbon gases at or near saturation in the interstitial waters of the near-bottom sediments will form hydrates. The gas can either be autochthonous, microbially produced gas, or allochthonous, catagenic gas from deeper sediments. Equilibrium conditions that stabilize hydrated sediments may be disturbed, for example, by continued sedimentation or by lowering of sea level. In either case, some of the solid gas-water matrix decomposes. Released gas and water volume exceeds the volume occupied by the hydrate, so the internal pressure rises--drastically if large volumes of hydrate are decomposed. Part of the once rigid sediment is converted to a gas- and water-rich, relatively low density mud. When the internal pressure, due to the presence of the compressed gas or to buoyancy, is sufficiently high, the overlying sediment may be lifted and/or breached, and the less dense, gas-cut mud may break through. Such hydrate-related phenomena can cause mud diapirs, mud volcanos, mud slides, or turbidite flows, depending on s diment configuration and bottom topography.